The quinacridone family of pigments and their pigmentary properties are well known. The pigmentary quinacridones include the parent compound, quinacridone, and various disubstituted quinacridones.
In addition, solid solution pigments of several combinations of quinacridones are known for their improved photochemical and thermal stability. U.S. Pat. No. 3,160,510, which is here incorporated by reference, describes the phenomenon of solid solutions, their X-ray diffraction patterns, and preparative procedures.
It is well known in the pigments art that unsubstituted and substituted quinacridone pigments can be prepared by the oxidation of the corresponding dihydroquinacridones. The product of such an oxidation, known as a crude quinacridone, is generally unsuitable for use as a pigment because of its large particle size. The crude pigment must be further processed in order to develop the requisite pigmentary properties, such as particle size, particle shape, polymorphic phase and tinctorial strength.
The crude quinacridone is commonly converted to a pigmentary form by milling the crude quinacridone with large quantities of inorganic salt and then extracting the resulting mill powder, or by dissolving the pigment in large quantities of concentrated sulfuric acid and drowning the solution into water (acid pasting). Since these multistep procedures generally require a diversity of operations conducted at elevated temperatures in acidic environments, simpler, more economical procedures for preparing pigmentary quinacridones are highly desirable.
Pigmentary quinacridone solid solutions are also generally prepared by laborious, multistep procedures. One method involves dissolving the crude components in concentrated sulfuric acid and then precipitating the solid solution by drowning into water under high turbulence followed by ripening at elevated temperatures. The pigmentary solid solutions of quinacridones have also been prepared by a variety of milling procedures.
The present invention relates to the discovery that pigmentary quinacridone solid solutions are prepared directly if mixed dihydroquinacridones, which correspond to quinacridones capable of forming a solid solution, are simultaneously co-oxidized in the presence of a sufficient quantity of a particle growth inhibitor to yield the pigmentary composition directly; without the need for additional particle size reducing after treatments. This is surprising because solid solutions are not formed if the oxidation is carried out in the absence of the particle growth inhibitor or if the dihydroquinacridones are not simultaneously co-oxidized.
The inventive process provides a simple, economically attractive and ecologically advantageous means for the direct manufacture of a variety of pigmentary quinacridone solid solutions. In addition, the approach used by the inventive process eliminates the current multistep, labor intensive procedures used for the manufacture of pigmentary/quinacridone solid solutions.
An additional aspect of this application relates to the discovery that greater than 1% of a phthalimidomethylquinacridone particle growth inhibitor does not inhibit the oxidation of dihydroquinacridones, if the oxidation is carried out in a sufficiently basic medium, for example, under the conditions needed to oxidize unsubstituted dihydroquinacridone to a polymorph other than 7-quinacridone, or under the conditions needed to oxidize the disubstituted dihydroquinacridones, especially 2,9-dichloro-6,13-dihydroquinacridone. Since it is known from U.S. Pat. No. 4,197,404 that the oxidation of .beta.-dihydroquinacridone to .gamma.-quinacridone was inhibited when more than 1 percent of o-carboxybenzamidomethylquinacridone, which is a hydrolysis product of 2-phthalimidomethylquinacridone, was utilized as a particle growth inhibitor, it is surprising that the oxidation of dihydroquinacridones to the corresponding quinacridones under the present reaction conditions is not inhibited by the quantity of phthalimidomethylquinacridone particle growth inhibitor sufficient to effect direct preparation of pigmentary products. This discovery permits the phthalimidomethylquinacridones to be used as particle growth inhibitors for the direct preparation of pigmentary .alpha.-quinacridone, as well as for the direct preparation of pigmentary quinacridone solid solutions.